In an LTE system, carrier aggregation (CA: carrier aggregation) for performing communication by simultaneously using a plurality of carriers is adopted, in which a predetermined bandwidth (20 MHz at the maximum) is used as a basic unit. In carrier aggregation, a carrier which is a basic unit is called a component carrier (CC: component carrier).
When CA is performed, a PCell (Primary cell) that is a reliable cell for ensuring connectivity and an SCell (Secondary cell) that is an appendant cell are set for the user apparatus UE. The user apparatus UE connects to a PCell first, and then, an SCell can be added as necessary. The PCell is a cell similar to an independent cell supporting RLM (Radio Link Monitoring) and SPS (Semi-Persistent Scheduling) and the like.
The SCell is a cell which is set in the user apparatus UE by being added to the PCell. Addition and deletion can be performed by RRC (Radio Resource Control) signaling. Since SCell is in an deactivated state right after it is set in the user apparatus UE, communication becomes available (scheduling becomes available) only by activating it.
As shown in FIG. 1, in CA up to Rel-10 of LTE, a plurality of CCs under the same base station eNB are used.
On the other hand, in Rel-12, this is further expanded so that Dual connectivity is proposed in which simultaneous communication is performed by using CCs under different base stations eNB to realize high throughputs (non-patent document 1). That is, in Dual connectivity, the UE performs communication simultaneously using radio resources of two physically different base stations eNB.
Dual connectivity is a kind of CA, and it is also referred to as Inter eNB CA (inter-base station carrier aggregation), in which Master-eNB (MeNB) and Secondary-eNB (SeNB) are introduced. FIG. 2 shows an example of Dual connectivity. In the example of FIG. 2, an MeNB communicates with the user apparatus UE by a CC#1, and the SeNB communicates with the user apparatus UE by a CC#2 so that Dual connectivity (to be referred to as DC hereinafter) is realized.
In DC, a cell group formed by cell(s) (one or a plurality of cells) under an MeNB is called MCG (Master Cell Group), and a cell group formed by cell(s) (one or a plurality of cells) under an SeNB is called SCG (Secondary Cell Group). An UL CC is set in at least one SCell in an SCG, and PUCCH is set in one of the at least one SCell. The SCell is called PSCell (primary SCell). In the following, there is a case where base stations such as MeNB and SeNB are collectively called an eNB.